Sanji meets a little mouse that has made his way into the ship and he befriends the lil guy. To the point where he speaks to him all the time and Sanji ends up confessing all of his feelings for Usopp to the mouse thinking he won't say a thing because- Because he is a mouse.

But it turns out the lil guy is also a little shit and he's a gossiper at heart. So he tells Chopper. Chopper is so nervous about this new information he ends up telling Robin. Robin mentions this to Franky because she finds it amusing and Franky is pretty much one of the closest to the sniper. He makes a comment about it to both Jinbe and Brook even if he tries to keep the secret because he is too dramatic and it's an accident. And since Brook loves to gossip too and has no filter either he talks about it with Nami. And if Nami finds out about a gossip, Zoro will too. And Zoro tells Luffy everything because they are basically married.

So now everybody knows Sanji has the biggest crush on Usopp because his pet mouse likes to see the world burn.